The present invention relates to wireless communications, and more particularly to wireless communications using different transmit diversity approaches in at least two adjacent time slots.
Information is typically transmitted from one location to another using only a single antenna in older wireless communications systems. For instance, in conventional cellular telephone systems, information from a base station to a given mobile terminal would be transmitted using a single antenna at the base station. While such single antenna approaches have been generally successful, it has be suggested that better performance may be achieved through the use of transmit diversity. Transmit diversity involves the transmission of information using more than one antenna. One reason for using transmit diversity is that the signal received at the mobile terminal from the second antenna may, in some circumstances, be stronger than that received from the first antenna due to path differences between the signals from the two antennas, even if the same overall transmit power is used. Several forms of transmit diversity have been proposed, with varying degrees of robustness and suitability for particular situations. While transmit diversity may have some theoretical advantages, transmit diversity has proven somewhat difficult to implement in practice, particularly in ways that allow so-called legacy mobile terminals (e.g., older mobile phones) to remain operational.
An approach to hybrid transmit diversity which involves the transmission of information using different forms of transmit diversity for at least two adjacent time slots is disclosed. In one aspect of the present invention, particularly suited to communications from a base station to mobile terminals, information is transmitted using a first transmit diversity technique during a first time slot and then information is transmitted using a second transmit diversity technique in the next subsequent time slot. For example, a base station may transmit information to mobile terminal X using delay diversity during time slot ts1 and then transmit information to mobile terminal Y using X space-time diversity during the next time slot, ts2.
In another aspect of the present invention, particularly suited to communications according to the standard known as Global System for Mobile Communication, most transmissions are made with delay diversity, but the logical channel known as the Frequency Correction Channel is transmitted with combined delay diversity and antenna hopping diversity. In some optional aspects, the antenna hopping diversity is accomplished using virtual antenna hopping (described below).
In yet another aspect of the present invention, particularly suited to communications according to the standard known as TIA/EIA-136, transmissions are made with delay diversity in a first time slot of a frame and with space-time diversity in a second time slot of the frame. Methods are disclosed below for accommodating the change in diversity techniques between the time slots that involve either transmitting dummy symbol(s) as part of the data field or stealing symbol(s) from the sync field to use in the data field.